Method for finishing a wood board

ABSTRACT

The invention relates to a method for finishing a wood board with an upper face and a lower face. A layer of liquid synthetic resin is first applied at least onto the upper face or the lower face, and at least one paper ply soaked with a synthetic resin is then applied. The structure is then pressed in a press under high pressure and at a high temperature, wherein the synthetic resin melts and is connected to the upper face or the lower face of the wood board. The invention is characterized in that the upper face and/or the lower face is provided while still having a press skin and in an ungrounded state, and the layer is applied so thinly that the liquid synthetic resin is completely drawn into the press skin, the layer is not actively dried, and the paper ply is positioned on the press skin.

FIELD OF THE INVENTION

The invention relates to a process for the finishing of a woodencomposite board with an upper side and an underside, in that firstly alayer of liquid synthetic resin is applied at least to the upper side orthe underside, then at least one paper ply impregnated with a syntheticresin is applied, and this structure is then pressed in a press underhigh pressure and at high temperature, where the resin melts and bondsto the wooden composite board.

DISCUSSION OF BACKGROUND INFORMATION

Such a process is known, for example, from EP 2 743 094 A1. Applyingsynthetic resin to the upper side of the carrier board is intended tomake it possible to use a decor paper that is impregnated on only oneside, because the liquid resin applied to the carrier board solubilisesthe pre-dried impregnating agent on the back side of the decor paper, sothat the decor paper adheres more quickly to the carrier board. Beforethe decor paper is applied, the synthetic resin layer applied in liquidform on the upper side must be dried.

In the wooden composite board disclosed in DE 10 2007 012 236 B4, acarrier layer of paper is first applied to the upper side. A decorativelayer is applied to this carrier layer, and then a sealing lacquer coatis applied to the decorative layer. The lacquer coat is then hardenedcompletely by means of electron beams and a structure is embossed in thelacquer coat.

Individual panels are then cut out of a large-sized laminate board sofinished, which panels can be used as a floor, wall or ceiling covering.The panels can be provided with a tongue-and-groove connection on theirside edges. A problem with laminates having a paper structure isdelamination, which is to be encountered wherever materials are joinedtogether layerwise with the addition of binder, heat and pressure.Delamination can in many cases be seen externally as blisters, or thepaper layer in a panel becomes detached from the board at the edges.

In order to rule out delamination, there has occasionally been a movetowards coating boards directly.

DE 197 51 115 A1 discloses a method of coating a panel, in which atleast one coloured layer is applied to the surface by means of aprinting method, in particular by means of screen printing. The surfacecan thereby be untreated, ground or pretreated, in particular lacquered.The applied coloured layer can finally be covered by coating with clearlacquer.

In DE 10 2004 026 739 A1 there is described a method in which anundercoat is applied by means of a roller to the upper side of thewooden composite board and dried. A first and second lacquer layer whichhardens under the action of UV light is then applied to the driedundercoat, hardening not being completed. Finally, a UV-hardenabletop-coat lacquer is applied to the first two lacquer layers, and thetopmost lacquer layer is then plastically deformed. Direct printing isvery expensive and must be carried out very carefully in order to avoidinclusions in the print structure. In addition, it is important thateach individual layer is dried sufficiently before the next layer isapplied in liquid form, in order to prevent colours from merging.

When direct printing technology is used, that is to say when theindividual layers are printed directly onto a carrier board, there isultimately a reduction in thickness of the finished laminate panelscompared to panels manufactured in the conventional manner owing to theomission of the paper plies. This leads to problems in continuousproduction, when the large-sized coated laminate boards are then dividedup to produce the panels. In order to ensure that individual panels donot fail to meet DIN 13329 because they are undersized in terms ofthickness, rigorous quality assurance must be implemented, which on theone hand slows down manufacture and on the other hand increases theproduction costs further. Ultimately, it also causes irritation for theretailer and/or end user, since the stacking height of direct-coatedboards is visibly different from that of boards coated with paper plies.

In the known board manufacture, HDF boards or MDF boards areconventionally used as the carrier board, the upper side of which isground off by about 0.3 mm. A press skin, also called a press patina orrotting layer, forms on the upper side and the underside of the carrierboard. This press skin forms during pressing of the pressed fibre matand is produced by the hot surface of the pressing plates or belts ofthe press. The press skin has a thickness of about 0.3 mm. Since thepress skin is ground off completely and about 0.1 mm of the corematerial is ground off in order to reduce the surface roughness on theupper side of the board prior to further coating, the boards must bemanufactured thicker by a corresponding grinding allowance, whichadversely affects the production costs. In order to produce a floorlaminate having a thickness of 6.0 mm which conforms to standards and isproduced by direct printing technology, the carrier board must have atleast 0.61 mm.

In EP 2 236 313, for example, it is described that the press skin mustbe ground off because the heat input in the region thereof during hotpressing is so high that the adhesive hardens too quickly, so that gluebridges break at least partly and make the applied layer sensitive. Thisbreaking of the glue bridges makes finished panels susceptible tolifting of the decorative and abrasion-resistant layers applied to thecarrier board. This delamination, which can occur even under normalloading and out of the order of magnitude typical for wooden compositematerials due to climate fluctuations, is not tolerated by the customer.

SUMMARY OF THE INVENTION

There is therefore a great need for abrasion-resistant laminate panelswhich are within the thickness tolerance found in the above-mentionedstandard. There is further a need for a resource-saving productionprocess in which extra costs arising from additional process stepsand/or additional materials are largely avoided.

Starting from this problem, a process for the finishing of a woodencomposite board that is coated with at least one paper ply is to be soimproved that a strong bond between the paper layer and the upper sideof the wooden composite board is produced, so that delamination isreliably avoided. In this case, it ideally ought to be possible to useconventional fully impregnated paper.

The problem is solved in the case of a process of the generic type inthat the upper side and/or the underside retain a press skin and areprovided without surface grinding, and the layer applied is so thin thatall of the liquid synthetic resin enters the press skin, no activedrying of the layer takes place, and the paper ply is placed onto thepress skin.

It has been shown, wholly surprisingly, that this application ofsynthetic resin, which is known per se from direct printing technology,leads to a substantial improvement in the adhesion of the paper ply tothe upper side and/or the underside of the wooden composite board.Because the paper, which is generally a decorative paper, is impregnatedwith a synthetic resin, it could be assumed that this better adhesionoccurs simply because more resin is used in the lamination. Thisassumption is not correct, however, because delamination phenomena areknown to occur even when the decorative paper is saturated with resinand a paper that is very absorbent is used, so that the amount ofsynthetic resin that enters the laminate structure is increased.Delamination cannot reliably be prevented with such a structure.

Tests have shown that the resin with which the paper ply is impregnateddoes not begin to dissolve, as it does in EP 2 743 094 A1. The syntheticresin drawn completely into the press skin is inactive here.

The improved adhesion is probably attributable to the fact that theupper side and/or the underside is provided without surface processing,that is to say untreated and with the full press skin. The syntheticresin applied in liquid form penetrates into the press skin and thusforms, together with the press skin, an optimum base for the decorativepaper applied subsequently.

The liquid synthetic resin can be applied by rolling, spreading,trowelling or spraying.

By omitting active drying, the manufacturing process is accelerated and,in addition, a significant energy saving is achieved.

Preferably, the liquid synthetic resin is applied in a quantity of from5 to 100 g/m², in particular from 5 to 50 g/m² and more particularlypreferably from 5 to 25 g/m².

The synthetic resin is preferably a melamine resin or a urea resin or amelamine-urea-resin mixture.

Good results have been achieved with a coating in which the ratio ofsynthetic resin to water is (60:40) with a tolerance of ±10%.

The synthetic resin can comprise additives, in particular hardenersand/or wetting agents and/or thermoplastic polymers.

The paper ply is preferably covered with a wear layer made of syntheticresin before the pressing procedure. This wear layer can be produced byan overlay paper impregnated with a synthetic resin. The paper ply ispreferably a decorative paper. The wear layer can, however, also beapplied in liquid form. It is also conceivable to scatter syntheticresin particles onto the paper ply for the wear layer.

The wooden composite board finished by the process according to theinvention corresponds to a conventional laminate having a paperstructure on the upper side, with the fundamental difference that theadhesion between the paper structure and the wooden composite board issignificantly improved by the previously applied liquid synthetic resinlayer. The wear layer can thus comprise abrasion-resistant particles,for example corundum, in order to increase the abrasion resistance ofthe laminate. A structure can be embossed in the wear layer duringpressing of the laminate structure. This structure can correspond to thedecoration (“synchronous pore”). The decoration can be a wood grain or atile effect. In the case of a wood grain, the structure then correspondsto the pattern of the grain, the knotholes or pores in the reproducedwood surface. In the case of a tile effect, the structure corresponds tothe joints.

The improvement achieved compared to a conventional laminate structurehaving paper plies can readily be seen in the figures. The figures eachshow an apparent density profile of a coated MDF board from the upperside (0 mm) to the middle of the wooden composite core (3 mm).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a laminate structure.

FIG. 2 shows a diagram.

FIG. 3 shows a board treated in accordance with the invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 shows the laminate structure with a standard MDF board, the upperside of which has been ground in order to remove the press skincompletely. A synthetic-resin-impregnated decorative paper ply has beenplaced on the ground upper side, and an overlay paper ply has beenplaced thereon and pressed. The delamination strength of the top layeris 2.0±0.1 N/m². The high apparent density of 1900 kg/m³ results on theupper side from the paper structure. The apparent density fallscontinuously towards the middle of the wooden composite core.

In the case of the diagram shown in FIG. 2, a standard MDF board with anunground upper side has been used, that is to say an MDF board with acomplete press skin, on which a synthetic-resin-impregnated decorativeply and an overlay paper ply have been placed and pressed. The abrasionresistance of the top layer is 1.5±0.3 N/m. The apparent density is herelower at the peak, but it is also formed by the paper layers. Theapparent density initially falls continuously, the increase at a depthof about 0.25 mm results from the press skin, which has not been removedfrom the upper side. Here there is a risk of delamination, because thebond between the paper structure and the wooden composite board is poor.

The board tested according to FIG. 3 is a standard MDF board treatedaccording to the invention, which was made available with an unprocessedupper side and coated with 50 g/m² melamine resin before asynthetic-resin-impregnated decorative paper and overlay paper ply wereplaced on and pressed. The abrasion resistance of the top layer was1.9±0.1 N/mm². It can be seen that the maximum apparent density reachesalmost the value of the standard board (FIG. 1), but this then fallscontinuously and, although the press skin was not removed, no furtherincrease in the apparent density is to be noted. As can be seen from acomparison of FIGS. 2 and 3, the defect which can lead to delaminationphenomena could largely be avoided, so that a largely closed curve shapeis established. Although the delamination strength of the top layer isat the level of the standard structure according to FIG. 1, it is to betaken into consideration that the production time is shortenedsignificantly simply because the press skin does not have to be groundoff, a cost saving is thereby achieved, and the region of the upper sidethat is to be ground off does not have to be compensated for beforehandby the introduction of more material during production of the MDF board.It will be seen that delamination is reliably avoided according to theinvention, and in some cases the resin-coating of the impregnated paperplies can also be reduced because the synthetic resin necessary forbonding is possibly provided beforehand by the introduction of syntheticresin into the press skin. The underside of the wooden composite boardcan be finished in exactly the same way as the upper side. The upperside and the underside can also be treated identically. Both sides arethen preferably finished at the same time.

The invention claimed is:
 1. A process for the finishing of a woodencomposite board with an upper side and an underside, comprising a layerof liquid synthetic resin applied at least to the upper side or theunderside, then at least one paper ply impregnated with a syntheticresin is applied, and the synthetic resin and the paper ply are thenpressed in a press under high pressure and at high temperature, wherethe synthetic resin melts and bonds to the upper side or the undersideof the wooden composite board, wherein the upper side and/or theunderside retain a press skin and are provided without surface-grinding,and the layer of liquid synthetic resin applied is such a thickness thatall of the synthetic resin enters the press skin, no active drying ofthe layer takes place, and the paper ply is placed onto the press skin;wherein the liquid synthetic resin is a melamine resin or a urea resinor a melamine-urea-resin mixture.
 2. The process as claimed in claim 1,wherein finishing process is carried out identically on both the upperside and the underside.
 3. The process as claimed in claim 1, whereinthe liquid synthetic resin is applied by rolling, spreading, trowellingor spraying.
 4. The process as claimed in claim 1, wherein a quantityapplied of the liquid synthetic resin is from 5 to 100 g/m².
 5. Theprocess as claimed in claim 4, wherein a quantity applied of the liquidsynthetic resin is from 5 to 50 g/m².
 6. The process as claimed in claim5, wherein a quantity applied of the liquid synthetic resin is from 5 to25 g/m².
 7. The process as claimed in claim 1, wherein the ratio of theliquid synthetic resin to water is (60:40) with a tolerance of ±10%. 8.The process as claimed in claim 1, wherein the liquid synthetic resincomprises additives.
 9. The process as claimed in claim 8, wherein theliquid synthetic resin comprises hardeners and/or wetting agents and/orthermoplastic polymers.
 10. The process as claimed in claim 1, whereinat least one paper ply is a decorative paper.
 11. The process as claimedin claim 1, wherein before the pressing procedure, the paper ply iscovered with a wear layer made of synthetic resin.
 12. The process asclaimed in claim 11, further comprising an overlay paper placed onto thepaper ply.
 13. The process as claimed in claim 1, wherein the woodencomposite board is untreated and has a full press skin.
 14. The processas claimed in claim 13, wherein the paper ply is produced by applying anoverlay paper impregnated with the synthetic resin.
 15. The process asclaimed in claim 13, wherein the paper ply is produced by applying aliquid synthetic resin.
 16. The process as claimed in claim 13, whereinthe paper ply is produced by applying scatter synthetic resin particlesonto the paper ply for a wear layer.
 17. The process as claimed in claim13, wherein the wooden composite board is MDF.
 18. The process asclaimed in claim 13, wherein a delamination strength of the paper plylayer is 2.0±0.1 N/m² and the upper side of the wooden composite boardwith the layer and the impregnated paper ply has a density of 1900kg/m³, which falls continuously towards a middle of the wooden compositecore.
 19. The process as claimed in claim 13, wherein liquid syntheticresin is a melamine resin.